The present invention relates to a method of and apparatus for irradiating an object. More particularly this invention concerns a system usable in a copying apparatus for irradiating a line of a master to be copied.
It is known to cause a beam of light to travel along a line of a master to be copied by projecting the beam of light from a fixed light source against a helical reflecting surface. This surface is formed so that the reflected light travels along the line to be copied.
It is known to form the mirror as a drum having a helical ridge one of whose side surfaces is reflective. In the simplest arrangement the side surfaces extend perfectly radially of the rotation axis of the drum and the helix runs at a 45.degree. pitch. Thus the light is reflected exactly perpendicularly by the mirror.
Such an arrangement has the considerable disadvantage that it is very difficult to make the mirror and machine this helical ridge on it.
For this reason the helical reflecting surface is usually formed so as not to extend exactly radially, by which is meant it is not formed of a family of lines extending radially of the axis. Thus the beam is reflected by the surface in a direction generally not lying in a plane orthogonal to the central axis of the helix which is also the axis about which the mirror is rotated. With the system described immediately above the light is incident upon the master or the copy carrier which is arranged in a plane parallel to the rotation axis of the scanning mirror at an angle which differs from the right angle. Thus image sharpness is considerably reduced and copy quality is similarly adversely affected.
In addition, when the beam is not deflected in a direction lying in a plane orthogonal to the central axis, the master or the copy carrier must be displaced in an axial direction with respect to the scanning mirror. Such displacement is disadventagous in many apparatuses, because it increases the size and the cost of the machine. Also, for symmetry reasons or reasons of beam direction this axial displacement is often undesirable.
An exemplary scanning system according to the prior art can be seen in German published patent specification No. 2,131,467 naming as inventor M. M. Beduchaud.